1. Field of the Invention
The present invention relates to overhead camshaft internal combustion engines, and more particularly to overhead camshaft internal combustion engines having molded plastic camshafts.
2. Description of the Related Art
Camshafts are used in an overhead camshaft internal combustion engine to control the movement of intake and exhaust valves associated with the combustion chamber. The camshaft is conventionally connected to a crankshaft through a connecting mechanism, such as a sprocket and belt assembly or a gear assembly, and rotates with the crankshaft to coordinate the movement of the intake and exhaust valves during the combustion cycle. Lobes disposed on the camshaft exert axial force on the intake and exhaust valves as the camshaft rotates. The orientation and shape of the lobes on the camshaft control the movement sequence of the intake and exhaust valves.
In some engine configurations, the camshaft is only partially disposed inside the engine cylinder head and extends through an opening on the cylinder head. The portion of the camshaft inside the cylinder head includes cam lobes for operating the intake and exhaust valves and the portion of the camshaft outside the cylinder head includes an assembly for connecting the camshaft with the crankshaft. The camshaft assembly is usually supported by a journal bearing in the cylinder head opening and a seal assembly is disposed between the cylinder head walls and the camshaft to provide a fluid tight seal between the camshaft surface and the cylinder head opening. The seal assembly usually includes a tip portion made of rubber or other similarly flexible material, which is in contact with the camshaft surface. The rubber tip portion in combination with the camshaft surface forms a fluid barrier for minimizing fluid leakage between the cylinder head opening and the camshaft surface.
Conventionally, the camshaft and the cam lobes thereon are formed of metallic materials. However, camshafts formed of metallic materials can be relatively difficult and costly to manufacture. The manufacture of the camshaft itself as well as the cam lobes requires precision machining equipment and methods. Also, the cuttiing of the camshaft to form cam lobes results in scrap material which is wasted. Thus, the requirement of precision machining and the relatively high cost of the metallic materials raise the cost of manufacturing metallic camshafts.
Manufacturing camshafts from molded plastic material is one alternative to manufacturing camshafts from metallic materials. There are a number of advantages associated with using a molded plastic camshaft. Plastic camshafts can be easily molded from relatively inexpensive materials using conventionally known techniques. Such molded plastic camshafts are easily manufactured since expensive precision machining equipment and methods are not required. Plastic camshafts are also lighter in weight than metallic camshafts. The lighter weight eases manufacturing requirements as well as handling of the finished product by the end user. Further, plastic camshafts operate more quietly than camshafts formed of metallic materials.
However, a problem associated with using plastic camshafts in the configuration where the camshaft extends through a cylinder head opening is the difficulty in maintaining a tight fluid seal at the interface between the engine cylinder head and the plastic camshaft surface. A plastic camshaft surface is relatively abrasive compared to the soft plastic material used at the tip portion of the seal assembly. Therefore, as the plastic camshaft rotates at high speed, the abrasive camshaft surface destroys the rubber tip portion of the seal in a relatively short period of time resulting in the loss of the fluid tight seal. Also, since the abrasive surface is not uniformly in contact with the rubber tip portion, the contact between the rubber tip portion and the camshaft surface initially produces a lower quality seal.
Additionally, plastic is not a good heat sink and is unable to easily dissipate the frictional heat generated by the rotational contact between the plastic camshaft and the seal assembly. The inability to dissipate the frictional heat results in a heat build up in the plastic camshaft. The heat build up may be high enough to damage both the plastic camshaft and the seal assembly, such as by warping, melting, etc.
Therefore, what is needed is a plastic camshaft for use in an internal combustion engine which provides a smooth contact surface and the qualities of good heat dissipation and durability to ensure that the plastic camshaft and associated components, such as the seal assembly, do not wear out under high heat and wear conditions, and thereby maintain a tight fluid seal between the cylinder head opening and the plastic camshaft.